Add null object, and update top-level API specification (#157)

* Add `null` object, and update top-level API specification

* Address review comments on singleton and duck typing `null`

* Add a free `isnull` function to the top-level namespace

* fix _eq__ typo

* Change `isnull` type annotation to `object`

---------

Co-authored-by: Marco Edward Gorelli <marcogorelli@protonmail.com>

Author: rgommers

spec/API_specification/dataframe_api/__init__.py

@@ -3,7 +3,7 @@
 """
 from __future__ import annotations
 
-from typing import Mapping, Sequence
+from typing import Mapping, Sequence, Any
 
 from .column_object import *
 from .dataframe_object import *
@@ -14,8 +14,9 @@
 
 __dataframe_api_version__: str = "YYYY.MM"
 """
-String representing the version of the DataFrame API specification to which the
-conforming implementation adheres.
+String representing the version of the DataFrame API specification to which
+the conforming implementation adheres. Set to a concrete value for a stable
+implementation of the dataframe API standard.
 """
 
 def concat(dataframes: Sequence[DataFrame]) -> DataFrame:
@@ -73,3 +74,48 @@ def dataframe_from_dict(data: Mapping[str, Column]) -> DataFrame:
     DataFrame
     """
     ...
+
+class null:
+    """
+    A `null` object to represent missing data.
+
+    ``null`` is a scalar, and may be used when constructing a `Column` from a
+    Python sequence with `column_from_sequence`. It does not support ``is``,
+    ``==`` or ``bool``.
+
+    Raises
+    ------
+    TypeError
+        From ``__eq__`` and from ``__bool__``.
+
+        For ``__eq__``: a missing value must not be compared for equality
+        directly. Instead, use `DataFrame.isnull` or `Column.isnull` to check
+        for presence of missing values.
+
+        For ``__bool__``: truthiness of a missing value is ambiguous.
+
+    Notes
+    -----
+    Like for Python scalars, the ``null`` object may be duck typed so it can
+    reside on (e.g.) a GPU. Hence, the builtin ``is`` keyword should not be
+    used to check if an object *is* the ``null`` object.
+
+    """
+    ...
+
+def isnull(value: object, /) -> bool:
+    """
+    Check if an object is a `null` scalar.
+
+    Parameters
+    ----------
+    value : object
+        Any input type is valid.
+
+    Returns
+    -------
+    bool
+        True if the input is a `null` object from the same library which
+        implements the dataframe API standard, False otherwise.
+
+    """

spec/API_specification/index.rst

@@ -5,6 +5,21 @@ API specification
 
 .. currentmodule:: dataframe_api
 
+The API consists of dataframe, column and groupby classes, plus a small number
+of objects and functions in the top-level namespace. The latter are:
+
+.. autosummary::
+   :toctree: generated
+   :template: attribute.rst
+   :nosignatures:
+
+   __dataframe_api_version__
+   isnull
+   null
+
+The ``DataFrame``, ``Column`` and ``GroupBy`` objects have the following
+methods and attributes:
+
 .. toctree::
    :maxdepth: 3